The present invention relates generally to a submersible pumping system and, more particularly, to a submersible pumping system for discharging a fluid reservoir such as the hull of a cargo ship or a land-based storage vessel containing liquified gases.
There are currently a number of methods of removing a submerged electrical motor driven pump from a reservoir which contains liquified gases or cryogenic materials such as natural gas, methane, butane, propane, ammonia, ethylene, or other liquid media whose leakage to the atmosphere must be controlled. These techniques involve the installation of a casing in the reservoir to allow the pump and motor unit to be guided to a seat at the bottom of the reservoir and an appropriate discharge column to carry the pumped liquid to the top of the reservoir for discharge to the downstream system. In most systems, the casing forms the discharge column for the pumped liquid. In order to prevent the escape of vapors from the pumped liquid from the reservoir when the pump is being installed or removed, some devices are incorporated to isolate the casing from the remainder of the reservoir.
The majority of the reservoirs in which this type of system is employed utilize a foot valve at the bottom of the casing, such as disclosed in U.S. Pats. Nos. 3,369,715; 3,876,120 and 4,080,106, assigned to the assignee of the present application. In these arrangements, in order to remove the pump and motor unit from the fluid transmitting casing, the casing is purged by an inert gas. Liquid in the reservoir cannot re-enter the casing because the valve at the bottom thereof is normally biased to a closed position. Because this system requires a mechanical valve to function at the bottom of the casing there is a remote possibility that the valve could jam in either the closed or open position and cause associated problems with either the removal or installation of the pump and motor unit. If the valve is jammed in the closed position, it would be impossible to have the liquid pumped from the reservoir.
One arrangement for alleviating the potential problem of a jammed foot valve is to provide an isolation chamber above the top of the reservoir which may be sealed off from the fluid transmitting casing therebelow and, hence, the reservoir by a suitable valve, such as a ball valve through which the pump and motor unit and its suspension cable may be drawn. Once the pump is lifted to the isolation chamber and the ball valve is closed, the chamber is purged to enable removal of the pump without venting flammable gases to the atmosphere. The suspension and lifting means for the pump and motor unit comprises a rigid lift pipe which contains the electrical cable for the motor of the pump. The pipe comprises a plurality of sections typically ten feet long and about four inches in diameter. The multi-section pipe must be pulled through a sealing gland at the top of the isolation chamber to lift the pump and motor unit. The pipe sections must be separated as they emerge from the top of the chamber, which also requires that the electrical conductors be disconnected at each separation point of the lift pipe. Thus, there are many mechanical and electrical disconnections that must be made which is inconvenient and time consuming. An additional problem exists with such a system because the cold pipe emanating from the sealing gland has frost forming on it as the pipe is exposed to atmospheric moisture. Unless the mechanical separation of the lift pipe is accomplished very quickly, the front build-up can cause problems with disconnecting the mechanical portions of the pipe. Also, the lift pipe must be drained, vented and purged as each section is removed from the reservoir. Another problem with this arrangment is that in the course of handlling of the pipe sections, it is quite easy to damage the outer surface thereof, thereby reducing if not eliminating the capability of the sealing gland to seal to the pipe.
Another submersible pumping system in which an isolation chamber for the pump and motor unit is located adjacent to the top of the roof of a reservoir is disclosed in U.S. Pat. No. 3,696,975. In this system, a flapper valve on the bottom of the isolation chamber is opened to allow the pump and motor unit to be lowered by a flexible lifting cable to a discharge housing adjacent to the bottom of the reservoir. A flexible electrical conductor cable extends from the top of the isolation chamber down to the motor of the pump and motor unit alongside the lift cable. The cables extend through separate sealing glands in the head plate of the isolation chamber. A purge gas is introduced into the assembly above the head plate and thus around the sealing glands. While this arrangement has the advantage that flexible cables are utilized which do not need to be disconnected in sections during pulling of the lift cable, the lift cable is typically formed of standard wire, which can damage the sealing gland and, hence, cause leakage in the system after several removals of the pump from the reservoir. Furthermore, because the lift cable and conductor cable which extend to the pump and motor unit run parallel to each other and are both connected to the pump and motor unit, if the unit rotates in the discharge housing at the bottom of the reservoir, which commonly occurs, the two cables may become twisted thus making it difficult if not impossible to pull the cables through their respective sealing glands when it is desired to raise the pump and motor unit.
Another arrangement used for removable submerged pumps in cryogenic fluid reservoirs is disclosed in U.S. Pat. No. 4,174,791. In this system, electrical cables are utilized which extend from the top of the reservoir to a discharge housing adjacent to the bottom of the reservoir where the conductors of the cables are connected to contacts in the housing. The pump and motor unit is lowered into the discharge housing so that contacts thereon will engage the contacts on the discharge housing thereby providing electrical connection between the electrical cables and the motor of the pump and motor unit. Thus, in this arrangement the electrical cables are not raised when the pump and motor unit is removed from the reservoir. The difficulty associated with this arrangement is that the electrical connection between the electrical cables and the motor is made in a purely remote fashion near the bottom of the reservoir. Should there be an interruption of either the electrical cables or the electrical connection, there is no way to economically repair the same because the cables and contacts are fixed adjacent to the bottom of the reservoir which is filled with liquid.
It is the purpose of the present invention to provide a unique removable submerged pumping system which allows the entire pump and electrical cables to be removed from the fluid transmitting casing and utilizes a top closure valve in the casing so that the potential problem of a malfunctioning foot valve is eliminated. The present invention provides the capability of having a continuous mechanical and electrical connection from the top of the fluid transmitting casing to the bottom thereof that goes through an effective seal in a manner that does not require disconnecting of separate electrical conduits nor does it require a separation of multiple lift pipes for removal of the pump from the reservoir.